Was 2.3 seconds. Now 1.4 seconds.

Next win would be not running a child process and refactoring main so
it could be called from tests easily. But that would also require
rewriting the errchk written in Perl. This appears to be the last user
of errchk in the tree.

Updates #17751

Change-Id: Id7c3cec76f438590789b994e756f55b5397be07f
Reviewed-on: https://go-review.googlesource.com/32754
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>